Birdsong has many features that make it a useful system for studying vertebrate learning. Song is a complex motor behavior learned in distinct phases during the course of a young animal's life. This learning is experience-dependent: it is influenced by exactly what and how much the bird hears. There are critical periods for song learning, and the neural circuit controlling song vocalization is discretely localized (Figure 31; for review, see Konishi 1989). The song behavior is sexually dimorphic; the male sings, whereas the female sings little if at all, and the brain song control system is similarly sexually dimorphic (Nottebohm and Arnold 1976; Gurney 1981). Steroid hormones influence both the adult song behavior and the sexual differentiation of the song system (Gurney 1981 1982). All of these characteristics of birdsong provide a unique opportunity to study how the brain changes during learning and development, and to investigate the role of experience in shaping these changes. © 1991.